290 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



ability of sequence, and relative and absolute thicknesses. Patterns 

 vary under different types of rainfall and different soil-moisture re- 

 gimes (Clock, 1955b). 



Strong evidence indicates that the trunk of a tree duplicates the 

 history and vicissitudes of growth as written in the branches. Of 

 course, the story in the trunk is continued longer and is perhaps more 

 synoptic in nature. Branches were studied to a great extent in the 

 present work not only because of the obvious advantage in manipula- 

 tions and measurements but also because their growth layers could 

 be dated in absolute fashion. If, by chance, branches carry a more 

 complete record of fluctuations in physiological activity than the 

 trunks do, then the study of branches becomes of great ecologic 

 importance. 



It is hoped that the present work will in some measure contribute 

 to a better understanding of tree growth, especially in the lower forest 

 border of southwestern United States, and that it has answered to 

 some extent the questions stated in the introduction. There remains 

 the task of applying the methods of exact dating, as defined herein, to 

 tree growth along a transect from the lower forest border into the 

 forest interior. An investigation of the fundamentals of tree growth 

 at the forest interior stands out as a desideratum of high order. When 

 does growth occur at the interior? Where on the plant body does 

 growth occur? Are partial growth layers ever found in normal trees 

 at the forest interior? These and other questions await analysis of 

 growth under the discipline of exact dating. Not only forest interiors 

 but also other forests, other forest borders, and other species, wher- 

 ever they may be, await patient investigation with promise of decisive 

 and perhaps unexpected results. 



